Add @ null annotations where Android Studio suggested it

That is, basically where the overriding function was missing an annotation from the base method.

Also apply renaming of emptyDescription to EMPTY_DESCRIPTION

extractor/src/main/java/org/schabi/newpipe/extractor/services/peertube/extractors/PeertubeChannelExtractor.java

@@ -138,7 +138,7 @@ public class PeertubeChannelExtractor extends ChannelExtractor {
     }
 
     @Override
-    public void onFetchPage(final Downloader downloader)
+    public void onFetchPage(@Nonnull final Downloader downloader)
             throws IOException, ExtractionException {
         final Response response = downloader.get(
                 baseUrl + PeertubeChannelLinkHandlerFactory.API_ENDPOINT + getId());

extractor/src/main/java/org/schabi/newpipe/extractor/services/peertube/extractors/PeertubeCommentsExtractor.java

@@ -23,12 +23,15 @@ import static org.schabi.newpipe.extractor.services.peertube.PeertubeParsingHelp
 import static org.schabi.newpipe.extractor.services.peertube.PeertubeParsingHelper.START_KEY;
 import static org.schabi.newpipe.extractor.utils.Utils.isNullOrEmpty;
 
+import javax.annotation.Nonnull;
+
 public class PeertubeCommentsExtractor extends CommentsExtractor {
     public PeertubeCommentsExtractor(final StreamingService service,
                                      final ListLinkHandler uiHandler) {
         super(service, uiHandler);
     }
 
+    @Nonnull
     @Override
     public InfoItemsPage<CommentsInfoItem> getInitialPage()
             throws IOException, ExtractionException {
@@ -84,6 +87,6 @@ public class PeertubeCommentsExtractor extends CommentsExtractor {
     }
 
     @Override
-    public void onFetchPage(final Downloader downloader) {
+    public void onFetchPage(@Nonnull final Downloader downloader) {
     }
 }

extractor/src/main/java/org/schabi/newpipe/extractor/services/peertube/extractors/PeertubeSearchExtractor.java

@@ -63,6 +63,7 @@ public class PeertubeSearchExtractor extends SearchExtractor {
         return Collections.emptyList();
     }
 
+    @Nonnull
     @Override
     public InfoItemsPage<InfoItem> getInitialPage() throws IOException, ExtractionException {
         return getPage(new Page(getUrl() + "&" + START_KEY + "=0&"

extractor/src/main/java/org/schabi/newpipe/extractor/services/peertube/extractors/PeertubeStreamExtractor.java

@@ -388,7 +388,8 @@ public class PeertubeStreamExtractor extends StreamExtractor {
     }
 
     @Override
-    public void onFetchPage(final Downloader downloader) throws IOException, ExtractionException {
+    public void onFetchPage(@Nonnull final Downloader downloader)
+            throws IOException, ExtractionException {
         final Response response = downloader.get(
                 baseUrl + PeertubeStreamLinkHandlerFactory.VIDEO_API_ENDPOINT + getId());
         if (response != null) {

extractor/src/main/java/org/schabi/newpipe/extractor/services/peertube/extractors/PeertubeTrendingExtractor.java

@@ -33,11 +33,13 @@ public class PeertubeTrendingExtractor extends KioskExtractor<StreamInfoItem> {
         super(streamingService, linkHandler, kioskId);
     }
 
+    @Nonnull
     @Override
     public String getName() throws ParsingException {
         return getId();
     }
 
+    @Nonnull
     @Override
     public InfoItemsPage<StreamInfoItem> getInitialPage() throws IOException, ExtractionException {
         return getPage(new Page(getUrl() + "&" + START_KEY + "=0&"

extractor/src/main/java/org/schabi/newpipe/extractor/services/youtube/ItagItem.java

@@ -13,6 +13,8 @@ import static org.schabi.newpipe.extractor.services.youtube.ItagItem.ItagType.VI
 import org.schabi.newpipe.extractor.MediaFormat;
 import org.schabi.newpipe.extractor.exceptions.ParsingException;
 
+import javax.annotation.Nonnull;
+
 public class ItagItem {
     /**
      * List can be found here
@@ -96,6 +98,7 @@ public class ItagItem {
         return false;
     }
 
+    @Nonnull
     public static ItagItem getItag(final int itagId) throws ParsingException {
         for (final ItagItem item : ITAG_LIST) {
             if (itagId == item.id) {

extractor/src/main/java/org/schabi/newpipe/extractor/services/youtube/extractors/YoutubeSearchExtractor.java

@@ -133,6 +133,7 @@ public class YoutubeSearchExtractor extends SearchExtractor {
         return !showingResultsForRenderer.isEmpty();
     }
 
+    @Nonnull
     @Override
     public List<MetaInfo> getMetaInfo() throws ParsingException {
         return YoutubeParsingHelper.getMetaInfo(